Selective laser sintering method, heat treatment method, metal powder, and shaped product

ABSTRACT

A selective laser sintering method alternately repeats a step of forming silicon alloy A2 powder (metal powder) in layers, the silicon alloy A2 powder made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt % and the balance of Fe and inevitable impurities, and a step of selectively applying a laser to the silicon alloy A2 powder formed in layers to melt and sinter the silicon alloy A2 powder.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-038168, filed on Feb. 27, 2015, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a selective laser sintering method that sinters metal powder with a laser and laminates it to form a desired three-dimensional shape, a heat treatment method, metal powder, and a shaped product.

2. Description of Related Art

Metal powder additive manufacturing is 3D shaping that does not require any mold, and it has attracted attention as one type of rapid prototyping (RP) that rapidly manufactures prototypes by using 3D CAD data. Recently, with the progress made in 3D printer technology, factors such as an increase in the accuracy of products, an increase in the speed of manufacture, and an increase in the types of materials have been added, and this processing method is now called additive manufacturing (AM) that is not limited to manufacturing prototypes, and that can directly manufacture end products. The applications which this method can be applied to have become widespread, and it can be applied not only to molds and machine parts but also customized medical parts like artificial joints and artificial crowns.

Because metal powder additive manufacturing is additive manufacturing that attaches material to a necessary part, it is possible to manufacture parts having shapes that have not been able to be manufactured by the existing processing methods (forging, casting, cutting etc.), such as an injection mold where an arbitrary cooling channel suitable for a product shape is placed inside the mold and an aerospace component like a fuel injection nozzle of a jet engine having a complex shape.

For example, Patent Literature 1 (Japanese Unexamined Patent Publication No. 2002-66844) discloses metal powder additive manufacturing that creates a plurality of slice data from 3D data of an object to be processed and applies a laser beam onto copper-nickel alloy powder based on the plurality of slice data to thereby sinter each layer and laminate them. However, because of constraints of device manufacturer, there are only a few types of materials that can be shaped by this method, which hinders widening of the applications which this method can be applied to.

In view of the foregoing, an object of the present invention is to provide a selective laser sintering method using a new type of steel, a heat treatment method, metal powder, and a shaped product.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a selective laser sintering method that alternately repeats a step of forming metal powder in layers, the metal powder being made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and a balance of Fe and inevitable impurities, and a step of selectively applying a laser to the metal powder formed in layers to melt and sinter the metal powder.

A particle size of the metal powder is 32 to 62 micrometers.

Energy density E is 31 to 124 [J/mm³].

Energy density E is 36 to 124 [J/mm³].

Energy density E is 50 to 124 [J/mm³].

Energy density E is 50 to 118 [J/mm³].

Also provided is a heat treatment method including a step of performing solution treatment on a shaped product shaped by the above-described selective laser sintering method, and a step of performing ageing treatment on the shaped product after the solution treatment.

According to a second aspect of the present invention, there is provided metal powder used in a selective laser sintering method of selectively applying a laser to the metal powder to melt and sinter the metal powder for lamination, wherein the metal powder is made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and a balance of Fe and inevitable impurities.

A particle size of the metal powder is 32 to 62 micrometers.

A shaped product shaped by the above-described selective laser sintering method is provided.

A shaped product heat-treated by the above-described heat treatment method is provided.

According to the invention, there is provided a selective laser sintering method using a new type of steel, a heat treatment method, metal powder, and a shaped product.

The above and other objects, features and advantages of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows photographs of silicon alloy A2 powder.

FIG. 2 shows a photograph of a sample.

FIG. 3 shows a graph representing the relationship between energy density E and relative density D.

FIG. 4 shows a graph representing the relationship between laser scanning speed v and relative density D.

FIG. 5 shows a graph representing the relationship between energy density E and relative density D.

FIG. 6 shows a graph representing the relationship between scanning pitch s and relative density D.

FIG. 7 shows photographs of the longitudinal section of a shaped sample after shaping.

FIG. 8 shows photographs of the longitudinal section of a sample after solution treatment.

FIG. 9 shows photographs of the longitudinal section of a sample after ageing treatment.

FIG. 10 shows a bar graph representing test results of a hardness test.

FIG. 11 is a view showing the size and shape of a sample of a tensile test.

FIG. 12 shows a photograph of a sample of a tensile test.

FIG. 13 shows a bar graph representing test results of a tensile test.

FIG. 14 is a flowchart of a selective laser sintering method and a flowchart of a heat treatment method.

DETAILED DESCRIPTION

1. Steel type

The applicant of the present invention has made a close investigation as to whether precipitation hardening stainless steel containing high Si and extremely low C can be employed as a steel type of metal powder to be used in a selective laser sintering method, and reports the investigation results in this specification. Hereinafter, the precipitation hardening stainless steel is referred to as “silicon alloy steel”. There are a plurality of component specifications of silicon alloy steel, and silicon alloy steel A2 shown in the following table 1 is selected in this shaping test. Further, powder of silicon alloy steel A2 is simply referred to as “silicon alloy A2 powder”. In the following table 1, the component specifications of silicon alloy steel and silicon alloy steel A2 are shown in comparison with the component specifications of typical SUS630. Note that silicon alloy steel A2 is a region where three phases, which are ferrite, austenite and martensite, coexist.

TABLE 1 C Si Mn Cu Ni Cr Mo Nb Fe Silicon alloy steel <0.10 2.0~9.0 0.05~6.0  0.5~4.0  1.0~24.0  6.0~28.0 0.2~4.0 0.03~2.0  Bal. Silicon alloy steel A2 <0.020 3.0~5.0 0.5~1.5 0.8~1.2 6.0~7.0 10.0~13.0 0.3~1.0 0.30~1.00 Bal. SUS630 <0.07 <1.0 <1.0 3.00~5.00 3.00~5.00 15.00~17.00 — 0.15~0.45 Bal.

Specifically, silicon alloy steel is high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and the balance of Fe and inevitable impurities.

Further, silicon alloy steel A2 is high silicon stainless steel containing C:less than 0.020, Si:3.0˜5.0, Mn:0.5˜1.5, Cu:0.8˜1.2, Ni:6.0˜7.0, Cr:10.0˜13.0, Mo:0.3˜1.0, Nb:0.30˜1.00 in wt %, and the balance of Fe and inevitable impurities.

C is an element that increases the strength of steel, and it is essential for typical high strength steel to contain a certain amount of C. However, in silicon alloy steel that contains a large amount of Si, because its strength is ensured by a peculiar metal structure produced by Si, it is not essential for it to contain C. Rather, C is an element that decreases the toughness of silicon alloy steel and adversely affects oxidation resistance and corrosion resistance. Thus, the content of C in silicon alloy steel is preferably as low as possible. The tolerable upper limit of the content of C in silicon alloy steel is 0.10%, and it is more preferably 0.05% or less. Further, the content of C in silicon alloy steel A2 is less than 0.020%.

Si is not only the primary element that gives steel its strength but also gives it heat resistance, oxidation resistance, corrosion resistance and softening resistance. Further, Si is an element that lowers the melting point of steel to increase the flowability and improve the castability thereof. When the content of Si is less than 2.0%, the improvement of the above properties is insufficient. On the other hand, because Si is a strong ferrite forming element, when the content thereof exceeds 9.0%, it is necessary to increase the amount of Ni or the like added in order to prevent the ferrite phase in the steel structure from becoming excessive, which would result in high material costs. Thus, the tolerable content of Si in silicon alloy steel is 2.0% to 9.0%, and further the tolerable content of Si in silicon alloy steel A2 is 3.0% to 5.0%.

Mn works as a deoxidizer of steel, and it is also an austenite forming element. Although Mn does not greatly affect the mechanical properties of silicon alloy steel, a Mn content of 0.05% or more is needed because it contributes to the densification and stabilization of the metal structure. If, on the other hand, the Mn content exceeds 6.0%, the corrosion resistance decreases. Thus, the tolerable content of Mn in silicon alloy steel is 0.05% to 6.0%, and further the tolerable content of Mn in silicon alloy steel A2 is 0.5% to 1.5%.

Cu is a component to be added to silicon alloy steel as needed. Cu is an element that contributes to the improvement of corrosion resistance (particularly, acid resistance) and precipitation hardening of silicon alloy steel. Further, Cu is an austenite forming element and contributes to the adjustment of the balance of the metal structure. To achieve these effects, the content of Cu is preferably 0.5% or more. However, because the Cu content exceeding 4.0% causes degradation of the hot workability of steel, the upper limit of the Cu content when added to silicon alloy steel is 4.0%. Thus, the tolerable content of Cu in silicon alloy steel is 0.5% to 4.0%. It is more preferably 2.0% or less. Further, the tolerable content of Cu in silicon alloy steel A2 is 0.8% to 1.2%.

Ni is an element that gives corrosion resistance (particularly, acid resistance), oxidation resistance and heat resistance to steel and it is essential to substantially maintain the duplex metal structure of steel in a good balance with Cr, which is described next. To obtain these effects, a

Ni content of 1.0% or more is required. If, on the other hand, the Ni content exceeds 24.0%, the austenite phase excessively increases, which causes the loss of not only the characteristics of duplex stainless steel but also reduces the cost efficiency of steel. Thus, the tolerable content of Ni in silicon alloy steel is 1.0% to 24.0%. Further, the tolerable content of Ni in silicon alloy steel A2 is 6.0% to 7.0%.

Cr is a component to ensure the basic characteristics of stainless steel, which are corrosion resistance (particularly, acid resistance), heat resistance and oxidation resistance. These properties are not sufficient if the Cr content is 6.0% or less. On the other hand, if the Cr content is more than 28.0%, the amount of Ni, which is required to substantially maintain the duplex metal structure of steel, increases so that the cost efficiency of steel is reduced. Thus, the tolerable content of Cr in silicon alloy steel is 6.0% to 28.0%. Further, the tolerable content of Cr in silicon alloy steel A2 is 10.0% to 13.0%.

Mo increases the corrosion resistance (acid resistance) and high-temperature strength to improve the creep resistance and further contributes to an increase in the toughness and wear resistance of silicon alloy steel. These effects are not sufficient if the Mo content is 0.2% or less. Because Mo is a ferrite forming element, if its content in silicon alloy steel becomes higher, the amount of an austenite forming element (Ni, Cu, Mn) added thereto needs to be increased. Further, Mo is an expensive element. In view of all these factors, the tolerable content of Mo in silicon alloy steel is 0.2% to 4.0%. Further, the tolerable content of Mo in silicon alloy steel A2 is 0.3% to 1.0%.

Nb is an element that is effective for increasing the case depth in aging treatment without degrading the toughness of silicon alloy steel. Further, Nb improves the intergranular corrosion resistance and weldability and also improves the strength of the silicon alloy steel. These effects are significant when the Nb content is 0.03% or more. If, on the other hand, the Nb content is more than 2.0%, the hot workability of silicon alloy steel is degraded, and the toughness is reduced. Thus, the tolerable content of Nb in silicon alloy steel is 0.03% to 2.0%, and more preferably 0.1% to 2.0%. Further, the tolerable content of Nb in silicon alloy steel A2 is 0.30% to 1.00%.

Silicon alloy steel including silicon alloy steel A2 contains, in addition to the above-described components, the balance of iron (Fe) and inevitable impurities. Note that it is preferred that the content of each of P and S, which are impurities, be 0.04% or less.

According to the above Table 1, silicon alloy steel A2 has higher contents of Si, Ni, Mo and Nb and lower contents of Cu and Cr compared with SUS630, respectively.

2. Silicon alloy A2 Powder

Silicon alloy A2 powder can be manufactured by, for example, gas atomizing. Gas atomizing is a method by which an alloy composed of desired components is dissolved, and then the molten metal flows out of a nozzle hole at the bottom of a tundish to produce a fine flow of the molten metal. Then, jet fluid composed of inert gas such as argon gas is sprayed against the flow of the molten metal to sequentially convert the molten metal flow into powder form by the energy of the jet fluid, and generated droplet is solidified as it is dropping to thereby produce alloy powder. FIG. 1 shows photographs of silicon alloy A2 powder manufactured by the gas atomizing. In this shaping test, silicon alloy A2 powder is classified so that the particles range in size from 32 to 62 micrometers (sieving method).

3. Selective Laser Sintering System

A selective laser sintering system used in this shaping test is EOSINT-M280 from EOS GmbH. The specifications of this selective laser sintering system are shown in the following table 2.

TABLE 2 Laser output P 250~350 W Laser scanning speed v   400~1600 mm/s Scanning pitch s 0.06~0.16 mm Lamination thickness t  50 micrometers Laser diameter 100 micrometers

The energy density E of laser sintering is defined by the following equation (1):

$\begin{matrix} {{{Equation}\mspace{14mu} 1\text{:}}\mspace{625mu}} & \; \\ {E = \frac{P}{v \times s \times t}} & (1) \end{matrix}$

4. Shaping Test

In this shaping test, the following four types of tests were conducted to determine whether silicon alloy A2 powder can be employed as the steel type of metal powder to be used in the selective laser sintering method.

(1) Energy Density Test

A change in the relative density of a sample when the energy density E was varied was examined. To be specific, the laser scanning speed v was varied to increase and decrease the energy density E, and then the scanning pitch s was varied to increase and decrease the energy density E.

(2) Structure Observation Test

The structure along the longitudinal section of a sample before and after solution treatment and ageing treatment was observed.

(3) Hardness Test

The hardness of a sample before and after solution treatment and ageing treatment was examined.

(4) Tensile Test

The maximum tensile strength of a sample before and after solution treatment and ageing treatment was examined.

Note that a specific component of silicon alloy A2 powder used in this shaping test was C:0.015, Si:3.45, Mn:0.96, Cu:1.12, Ni:6.7, Cr:10.8, Mo:0.39 in wt %, and the balance of Fe and inevitable impurities.

4.1. Energy Density Test

First, a method of calculating the relative density D is described. Specifically, the density of a sample (Archimedes method) is calculated by the following equation (2). The porosity P (note that the density of a pore is regarded as 0) of a sample is calculated by the following equation (3). The relative density D of a sample is calculated by the following equation (4). Note that, in the following equations (2) to (4), V is the overall volume of a sample, V′ is a pore volume, p is the true density (7.61 g/cm³), pt is the density of a sample, pw is the density of water, Min-air is the weight of a sample in air, Min-water is the weight of a sample in water, P is the porosity of a sample, and D is the relative density of a sample.

$\begin{matrix} {{{Equation}\mspace{14mu} 2\text{:}}\mspace{625mu}} & \; \\ {{M_{{in} - {air}} = {V \times \rho_{t}}}{M_{{in} - {water}} = {{V \times \rho_{t}} - {V \times \rho_{w}}}}{\rho_{t} = {\frac{M_{{in} - {air}}}{M_{{in} - {air}} - M_{{in} - {water}}} \times \rho_{w}}}} & (2) \\ {{{Equation}\mspace{14mu} 3\text{:}}\mspace{625mu}} & \; \\ {{{\rho_{t} \times V} = {{\rho_{t} \times \left( {V - V^{\prime}} \right)} + {0 \times V^{\prime}}}}{P = {{\frac{V^{\prime}}{V} \times 100} = {\frac{\rho - \rho_{t}}{\rho} \times 100(\%)}}}} & (3) \\ {{{Equation}\mspace{14mu} 4\text{:}}} & \; \\ {D = {{100 - P} = {\frac{\rho_{t}}{\rho} \times 100}}} & (4) \end{matrix}$

In the energy density test, a base plate was formed first, and a cylindrical sample with a diameter of 8 millimeters and a height of 15 millimeters was formed on the base plate as shown in FIG. 2. Each sample stands upright in the direction of lamination. After shaping, each sample was separated from the base plate, and the relative density of each sample was measured. The height of each separated sample was 12 millimeters.

The following table 3 shows test results when the laser scanning speed v was varied. In the following table 3, “Appearance” is a photograph of the upper end face of the shaped sample. Further, “Shaping stopped” in the following table 3 means that shaping was stopped because a sample was molten and could not keep its cylindrical shape. Note that the scanning pitch s was 0.1 mm, the thickness t of lamination was 50 micrometers, and the laser diameter was 100 micrometers. FIG. 3 shows the relationship between the energy density E and relative density D in the following table 3. In FIG. 3, the horizontal axis indicates the energy density E, and the vertical axis indicates the relative density D. A result when the laser output P was 250 [W] is indicated by an outline rhombus, a result when the laser output P was 300 [W] is indicated by an outline circle, and a result when the laser output P was 350 [W] is indicated by an outline triangle.

It is evident from the above Table 3 and FIG. 3 that (1) a sample cannot be shaped in a desired shape when the energy density E is 125 [J/mm³] or more, (2) a sample can be shaped without any problem when the energy density E is 31 [J/mm³], (3) there is a certain relationship between the energy density E and the relative density D, and (4) the relative density D begins to be saturated when the energy density E is 36 [J/mm³] or more. From these findings, it is found that a sample in a desired shape can be shaped without any problem when the energy density E is in the range of 31 to 124 [J/mm³]. Further, the energy density E is preferably in the range of 36 to 124 [J/mm³]. In this range, a high density sample with the relative density D of 95 [%] or more can be shaped. Further, the energy density E may be 36 to 118 [J/mm3].

Further, the energy density E is more preferably in the range of 50 to 124 [J/mm³]. In this range, a high density sample with the relative density D of approximately 98 [%] or more can be shaped. Further, the energy density E may be 50 to 118 [J/mm³].

FIG. 4 shows the relationship between the laser scanning speed v and the relative density D in the above table 3. In FIG. 4, the horizontal axis indicates the laser scanning speed v, and the vertical axis indicates the relative density D. A result when the laser output P was 250 [W] is indicated by an outline rhombus, a result when the laser output P was 300 [W] is indicated by an outline circle, and a result when the laser output P was 350 [W] is indicated by an outline triangle. It is evident from the above Table 3 and FIGS. 3 and 4 that the highest relative density D is obtained when the laser output P is 300 or 350 [W] and the laser scanning speed v is 800 [mm/s].

The following table 4 shows test results when the scanning pitch s was varied. In the following table 4, “Appearance” is a photograph of the upper end face of the shaped sample. Further, “Shaping stopped” in the following table 4 means that shaping was stopped because a sample was molten and could not keep its cylindrical shape. Note that the laser scanning speed v was 800 [mm/s], the thickness t of lamination was 50 micrometers, and the laser diameter was 100 micrometers. FIG. 5 shows the relationship between the energy density E and the relative density D in the following table 4. In FIG. 5, the horizontal axis indicates the energy density E, and the vertical axis indicates the relative density D. A result when the laser output P was 300 [W] is indicated by an outline rhombus, and a result when the laser output P was 350 [W] is indicated by an outline circle.

It is evident from the above Table 4 and FIG. 5 that (1) a sample cannot be shaped in a desired shape when the energy density E is 125 [J/mm³] or more, (2) a sample can be shaped without any problem when the energy density E is 47 [J/mm³], and (3) there is a certain relationship between the energy density E and the relative density D. From these findings, it is found that a sample in a desired shape can be shaped without any problem when the energy density E is in the range of 47 to 124 [J/mm³]. Note that the graph of FIG. 5 is substantially similar to the graph of FIG. 3.

FIG. 6 shows the relationship between the scanning pitch s and the relative density D in the above table 4. In FIG. 6, the horizontal axis indicates the scanning pitch s, and the vertical axis indicates the relative density D. A result when the laser output P was 300 [W] is indicated by an outline rhombus, and a result when the laser output P was 350 [W] is indicated by an outline circle.

It is evident from the above Table 4 and FIGS. 5 and 6 that the highest relative density D is obtained when the laser output P is 300 [W] and the scanning pitch s is 0.08 [mm].

Further, taking the above Tables 3 and 4 and FIGS. 3 to 6 into consideration as a whole, the energy density E is most dominantly involved in the relative density D. Accordingly, it can be stated that, to obtain a desired relative density D, it is important to manage the energy density E with particular attention.

4.2. Structure Observation Test

In this structure observation test, a structure observation was conducted on a shaped sample before any treatment, a sample after solution treatment, and a sample after solution treatment and ageing treatment to examine a change in the structure before and after solution treatment and ageing treatment. The purpose of the structure observation test is to see whether or not there is a difference between microstructure in the diameter direction and that in the height direction and to see whether three phases, ferrite, austenite and martensite, coexist. The conditions of the test were as follows.

Object to be tested: an object to be observed was a sample shaped with the selective laser sintering system at the laser output P of 300 [W], the laser scanning speed v of 800 [mm/s], and the scanning pitch s of 0.08 [mm]. The size of the sample and the procedure of shaping were the same as those in the energy density test described above.

Test method: a sample was cut using a fine cutter to obtain a longitudinal section thereof, and the longitudinal section was filed with resin. Next, the longitudinal section was ground sequentially using carbon Mac papers #80, #220, #600, #1200 and #2000. Then, the longitudinal section was burnished sequentially using diamond paste with a size of 3 micrometers and 1 micrometer. After that, the sample was immersed in marble liquid to etch the longitudinal section. Then, the structure of the longitudinal section was observed using an optical microscope (with a magnification of 50 to 100).

Solution treatment conditions: the sample began to be heated starting at a room temperature, maintained at 1050° C. for 20 minutes, and then water-cooled.

Ageing treatment conditions: the sample began to be heated starting at a room temperature, maintained at 480° C. for 7 hours, and then air-cooled.

FIG. 7 shows photographs of the longitudinal section of a shaped sample before solution treatment and solution treatment. As shown in FIG. 7, the microstructure (laminated structure) similar to thermal spraying is seen all over the longitudinal section. Further, the coexistence of three phases, which are ferrite, austenite and martensite, is found. Further, it is seen that the proportion of martensite slightly differs between the inward and the outward of the longitudinal section.

FIG. 8 shows photographs of the longitudinal section of a sample after solution treatment. As shown in FIG. 8, martensite is substantially all over the longitudinal section, and a slight amount of residual austenite is seen. The laminated structure has disappeared, and the sample has a substantially uniform metal structure.

FIG. 9 shows photographs of the longitudinal section of a sample after solution treatment and ageing treatment. As shown in FIG. 8, the residual austenite has substantially disappeared.

4.3. Hardness Test

A hardness test was conducted on a shaped sample before any treatment, a sample after solution treatment, and a sample after solution treatment and ageing treatment to examine a change in the hardness before and after solution treatment and ageing treatment. The conditions of the test were as follows.

Object to be tested: an object to be tested was the longitudinal section of a chuck of a sample, which is an object to be tested by a tensile test described later.

Test conditions: a low-load Vickers hardness test was conducted. A test force was 0.3 kgf, a loading time was 4.0 seconds, a retention time was 15.0 seconds, a unloading time was 4.0 seconds, an approach speed was 60 micrometers per second, and the number of tests was 8 to 12 for each sample.

FIG. 10 shows test results. In FIG. 10, “shaped” indicates a shaped sample before any treatment, “solution” indicates a sample after solution treatment, and “ageing” indicates a sample after solution treatment and ageing treatment. The vertical axis indicates Vickers hardness HV. In FIG. 10, measurement results are represented by box plots, and the average is represented by a bar chart. For reference, the catalogue values of a casted product of silicon alloy steel A2 are represented by horizontal bands. “Catalogue value (solution)” is the catalogue value of a casted product of silicon alloy steel A2 after solution treatment. “Catalogue value (ageing)” is the catalogue value of a casted product of silicon alloy steel A2 after solution treatment and ageing treatment.

It is obvious from FIG. 10 that a sample shaped with silicon alloy A2 powder has substantially the same Vickers hardness as a casted product of silicon alloy steel A2. Note that maraging steel after ageing treatment has Vickers hardness of HV513. Thus, it can be stated that a sample that has been shaped with silicon alloy A2 powder and undergone solution treatment and ageing treatment has the Vickers hardness that substantially equals that of maraging steel after ageing treatment.

4.4. Tensile Test

A tensile test was conducted on a shaped sample before any treatment and a sample after heat treatment to examine the maximum tensile strength of each sample.

Object to be tested: an object to be observed was a sample shaped with the selective laser sintering system at the laser output P of 300 [W], the laser scanning speed v of 800 [mm/s], and the scanning pitch s of 0.08 [mm].

Number of samples: 24 in total: 4 round bars (vertical, with no heat treatment), 4 round bars (vertical, with heat treatment), 4 round bars (horizontal, with no heat treatment), 4 round bars (horizontal, with heat treatment), 4 test bar shapes (with no heat treatment) and 4 test bar shapes (with heat treatment). Note that “with heat treatment” means that the sample is cutting is performed after the solution treatment, and then ageing treatment is performed after that treatment. “Vertical” indicates a sample shaped in an orientation that stands in the direction of lamination, and “horizontal” indicates a sample shaped in an orientation that is orthogonal to the direction of lamination.

Sample shape: As shown in FIGS. 11 and 12.

Solution treatment conditions: the sample began to be heated starting at a room temperature, maintained at 1050° C. for 20 minutes, and then water-cooled.

Ageing treatment conditions: the sample began to be heated starting at a room temperature, maintained at 480° C. for 7 hours, and then air-cooled.

Testing machine: INSTRON MODEL 4206

Tension speed: 1 mm/min

Detection of Strain: Strain Gauge Was Used

FIG. 13 shows test results. In FIG. 13, the lower bar on the left side of each bar pair in the graph is the one not subjected to the heat treatment, and the higher bar on the right side is the one subjected to the heat treatment. “Catalogue value (ageing)” is the catalogue value of a casted product of silicon alloy steel A2 after solution treatment and ageing treatment. It is obvious from FIG. 13 that the orientation of each sample with respect to the direction of lamination does not substantially affect the maximum tensile strength and that the sample after heat treatment has substantially the same maximum tensile strength as that of a casted product of silicon alloy steel A2 after heat treatment. Note that maraging steel after ageing treatment has the maximum tensile strength of 1890 MPa. Thus, it can be stated that a sample that has been shaped with silicon alloy A2 powder and undergone heat treatment has the maximum tensile strength that substantially equals that of maraging steel after ageing treatment.

According to the test results described above, it was concluded that (1) silicon alloy A2 can be employed as metal powder to be used in the selective laser sintering method, (2) a shaped product produced by selective laser sintering using silicon alloy steel A2 has substantially the same mechanical quality as that of a casted product of silicon alloy steel A2, and (3) a shaped product produced by selective laser sintering using silicon alloy steel A2 can properly serve as a replacement for a commercially available maraging steel.

Finally, a shaping method using silicon alloy A2 powder is described hereinafter referring to FIG. 14.

First, 2D CAD data (slice data) is created by slicing 3D CAD data of a shaped product (S90). Next, the selective laser sintering system forms silicon alloy A2 powder in layers (S100). Then, the selective laser sintering system selectively applies a laser to the silicon alloy A2 powder to make it molten and sintered in a desired shape (S110). After that, the selective laser sintering system determines whether a shaped product has been completed (S120), and when it determines that the shaped product has not been completed (No in S120), the selective laser sintering system returns the process to S100. On the other hand, when it determines that the shaped product has been completed (Yes in S120), the selective laser sintering system ends the shaping. Then, solution treatment is performed on the shaped product at a specified temperature for a specified period of time (S130). Further, ageing treatment is performed on the shaped product after the solution treatment at a specified temperature for a specified period of time (S140). A shaped product that is comparable to maraging steel after heat treatment is thereby obtained.

The above-described embodiment has the following features.

A selective laser sintering method alternately repeats (S120) a step (S100) of forming silicon alloy A2 powder (metal powder) in layers, the silicon alloy A2 powder made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and the balance of Fe and inevitable impurities, and a step (S110) of selectively applying a laser to the silicon alloy A2 powder formed in layers to melt and sinter the silicon alloy A2 powder.

It is preferred that the particle size of silicon alloy A2 powder be 32 to 62 micrometers.

In the above-described selective laser sintering method, the energy density E is 31 to 124 [J/mm³].

It is preferred that the energy density E be 36 to 124 [J/mm³].

It is more preferred that the energy density E be 50 to 124 [J/mm³].

It is further preferred that the energy density E be 50 to 118 [J/mm³].

The lower limit of the energy density E is any one of 31, 36, 38, 42, 43, 44, 47, 50, 54, 55, 58, 60, 63, 70, 73, 75, 83, 88, 94, 100, 109 and 118, and the upper limit of the energy density E is any one of 36, 38, 42, 43, 44, 47, 50, 54, 55, 58, 60, 63, 70, 73, 75, 83, 88, 94, 100, 109, 118 and 124.

A heat treatment method includes a step (S130) of performing solution treatment on a shaped product shaped by the above-described selective laser sintering method and a step (S140) of performing ageing treatment on the shaped product after the solution treatment.

Also provided is metal powder that is used in a selective laser sintering method that selectively applies a laser to metal powder to melt and sinter the metal powder for lamination, wherein the metal powder is made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and the balance of Fe and inevitable impurities.

The particle size of the metal powder is 32 to 62 micrometers.

Further, a shaped product shaped by the above-described selective laser sintering method is provided.

Furthermore, a shaped product heat-treated by the above-described heat treatment method is provided.

From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 

1. A selective laser sintering method comprising: alternately repeating a step of forming metal powder in layers, the metal powder being made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and a balance of Fe and inevitable impurities; and a step of selectively applying a laser to the metal powder formed in layers to melt and sinter the metal powder.
 2. The selective laser sintering method according to claim 1, wherein a particle size of the metal powder is 32 to 62 micrometers.
 3. The selective laser sintering method according to claim 1, wherein energy density E calculated by following equation is 31 to 124 [J/mm³] $E = \frac{P}{v \times s \times t}$ where P is an output of the laser, v is a scanning speed of the laser, s is a scanning pitch of the laser, and t is a lamination thickness of one layer of the metal powder.
 4. The selective laser sintering method according to claim 3, wherein the energy density E is 36 to 124 [J/mm³].
 5. The selective laser sintering method according to claim 3, wherein the energy density E is 50 to 124 [J/mm³].
 6. The selective laser sintering method according to claim 3, wherein the energy density E is 50 to 118 [J/mm³].
 7. A heat treatment method comprising: a step of performing solution treatment on a shaped product shaped by the selective laser sintering method according to claim 1; and a step of performing ageing treatment on the shaped product after the solution treatment.
 8. Metal powder used in a selective laser sintering method that selectively applies a laser to metal powder to melt and sinter the metal powder for lamination, wherein the metal powder is made of high silicon stainless steel containing C:less than 0.10, Si:2.0˜9.0, Mn:0.05˜6.0, Cu:0.5˜4.0, Ni:1.0˜24.0, Cr:6.0˜28.0, Mo:0.2˜4.0, Nb:0.03˜2.0 in wt %, and a balance of Fe and inevitable impurities.
 9. The metal powder according to claim 8, wherein a particle size of the metal powder is 32 to 62 micrometers.
 10. A shaped product shaped by the selective laser sintering method according to claim
 1. 11. A shaped product heat-treated by the heat treatment method according to claim
 7. 